Last year, Despicable Me 2 alone grossed $143.1 million in its first five days after opening on the Wednesday before the Fourth of July, one of the most lucrative holiday stretches of the year in terms of moviegoing. This year, the three new films making a major play eked out a combined $61.4 million during the same period — more bad news for an already miserable summer.

Heading into the holiday, summer box-office revenue was down 15 percent in North America from last year, one of the worst declines in recent memory. Now it's behind by 19.3 percent, or $2.3 billion vs. $2.8 billion, a difference of $500 million, according to Rentrak.

What's behind the summer drought? Hollywood studio executives and box-office observers blame a lack of megagrossing tentpoles, a dearth of doubles and triples and no huge animated family film. In other words, a number of films have underwhelmed (or bombed), including Fourth of July R-rated comedy Tammy, which posted a five-day debut of $32.9 million, Melissa McCarthy's lowest recent opening. (As a way of comparison, fellow R-rated comedy Neighbors launched to nearly $50 million in May).

Revenue for Fourth of July weekend hit only $130 million, down 44 percent from last year's $229.8 million haul and the lowest in 16 years. Granted, the holiday fell on a Friday this year — a disadvantage — but revenue managed to reach $160.2 million in 2008, the last time the Fourth was a Friday. One reason for the dramatic downturn is that no big tentpole rolled out, probably because no one wanted to open in the wake of Transformers: Age of Extinction, which debuted June 27.

Most Hollywood studios have come to rely heavily on big-budget tentpoles to anchor their summer slates, and this year is no different. But it will likely be the first summer since 2001 that no event title reaches $300 million at the North American box office, putting even more pressure on international results. In 2013, the season's first movie, Iron Man 3, took in $409 million domestically. The Amazing Spider-Man 2, which kicked off this summer, struggled to get to $200 million.

"That's a $200 million hole right off the bat," says one studio executive. "Then you have a string of disappointments. Take your pick."

In terms of top performers, Fox's X-Men: Days of Future Past is the summer's top-grossing film domestically at $227.1 million, as well as worldwide at $724.7 million, a franchise best. Paramount's Transformers: Age of Extinction will likely eclipse Days of Future Past domestically, having already earned nearly $175 million in its first 10 days in release, but it's trailing the last two films in the Transformers franchise.

"It's a product-driven business," says Fox domestic distribution chief Chris Aronson, whose studio boasts some of the summer's biggest success stories. In addition to Days of Future Past, they include The Fault in Our Stars, a low-budget movie that has earned $115.9 million domestically.

Fault is the sort of midrange film that box-office analyst Phil Contrino says is noticeably missing this summer, at least so far. "A summer of doubles and triples, or movies grossing between $80 million and $150 million, can make all the difference. We haven't had enough of those," he says.

Adam Sandler's 2013 summer comedy, Grown Ups 2, for instance, earned $133.7 million; his 2014 summer comedy, Blended, has earned just $43.8 million to date. And Seth MacFarlane's A Million Ways to Die in the West grossed roughly $42 million, compared to $218.8 million for MacFarlane's 2012 summer offering, Ted.

The two comedies that have worked domestically this summer are 22 Jump Street, earning $158.9 million to date, and Neighbors ($148 million).

The family marketplace is likewise struggling sans a Despicable Me 2 and Monsters University, which last summer grossed $368.5 million and $268.5 million, respectively, in North America.

This summer's offerings are light on comparison, save for Disney's live-action fairy tale, Maleficent, which is the No. 2 title of the season with $213.9 million in domestic ticket sales. (It has grossed north of $600 million globally.) DreamWorks Animation's How to Train Your Dragon 2 has been a disappointment, grossing $140 million to date. The only other studio animated title left to open is Disney's Planes: Fire and Rescue (July 18).

Relativity Media hoped to ignite the family marketplace with its live-action alien-robot movie, Earth to Echo, but it only debuted to $13.5 million over the long July Fourth holiday.

Aronson and other studio executives say the summer will certainly end down, but they are hopeful that the current gap will be narrowed by Labor Day, even without Fast & Furious 7 and Jupiter Ascending, both of which were originally supposed to open in July.

Contrino agrees. "There's Dawn of the Planet of the Apes, Sex Tape, Lucy, Guardians of the Galaxy and Teenage Mutant Ninja Turtles, which could do great with families. A lot of these could surprise. It's a little premature to be too pessimistic."